A screw of the general type mentioned above is described in DE 33 35 092 A1. It has proven very successful in practice, because a high unscrewing torque is achieved with a low screwing-in torque. In the case of this known screw, at least in a partial region of the thread, the outer thread edge extends in a wave form in the radial direction with a specific amplitude between wave crests with the thread height and wave troughs with a height reduced by the amplitude. In this case, the thread has, at least in the region of one of its flanks, in the region of the wave troughs of the thread edge indentations which interrupt the surface of the flank and the outer delimitation of which is the thread edge. In the regions of the wave crests of the thread edge that are not interrupted by indentations, a first apex angle is formed between the flanks extending in a straight line between the lowest point of the thread on the core and the thread edge, while a second, greater apex angle is formed in the lowest regions of the wave troughs. The thread extends up to the end of the screw tip, it being configured with the indentations and the waved thread edge from the screw tip, at least over the first adjoining turn of the thread. As a result, the tip acts as a kind of abrasive tool, the thread forming taking place directly at the tip of the screw, so that reliable centering and engagement with the workpiece are provided immediately when the screw is applied. In the case of this known screw, the indentations are formed symmetrically in relation to the center line of the waved thread edge as symmetrical paraboloids.
EP 0 394 719 B1 describes a similar thread-forming screw, in which however indentations on the flanks are formed asymmetrically in such a way that their front flank faces, in the screwing-in direction, extend more steeply than the rear flank faces, in the screwing-in direction. As a result, a further reduction of the screwing-in torque is achieved with at the same time an increase in the unscrewing torque. When screwing in, the resistance is less as result of the flatter configuration of the rear parabola parts in the screwing-in direction, whereas the unscrewing of the screw is made more difficult on account of the steeper arrangement of the parabola faces lying at the front in the screwing-in direction.
The present invention is based on the object of improving a screw of the generic type described above in such a way that the screwing-in torque is further reduced. At the same time, the screw is intended to be designed universally for screwing into various materials including softer materials, such as wood and the like, without pre-drilling and consequently automatically forming a hole, and harder materials, for example plastics and metals, into a pre-drilled hole.
The screw of this invention includes at least one of the two flanks of the thread being formed concavely in the region between the shank core and the thread edge, seen in radial profile, in such a way that the apex angle is less than a flank angle enclosed between imaginary straight flank lines defined in each case by a lowest point of the thread and the thread edge. Consequently, according to the invention, the apex angle is smaller than in the prior art, resulting in a more slender thread profile, so that the tapping torque when screwing in is favorably induced, in that the thread more easily forms a counter-thread in the respective material with material displacement, i.e. substantially without chips being formed. However, in spite of the slenderness of the thread profile, good mechanical strength is ensured by the thread profile according to the invention, because the lowest point of the thread is configured with a relatively great width.
In an advantageous configuration of the invention, the thread may be formed (in a way corresponding to the aforementioned prior art) with a waved thread edge and indentations on at least one flank, a more slender, second apex angle also being formed in the region of the wave troughs. In this case, an angular difference between the first and second apex angles should be as small as possible or even zero, i.e. the second apex angle in the region of the wave troughs and the indentations should also be as small as possible, in order to keep the tapping torque low by the slender profile shape. A continuous transition, virtually without any edge, between the thread flanks and the indentations is also advantageous here.
In addition or as an alternative to the configuration described above, it is envisaged to vary the size of the amplitude of the waved thread edge in dependence on different intended uses of the screw. For use for screwing into softer materials, such as wood or other fibrous materials and composite materials, the amplitude of the waved thread edge is approximately 0.2 to 0.4 times the thread height. The softer or more yielding the material is, the greater the amplitude can be (and vice versa). For use for screwing into harder materials, in particular plastics or metals, it is provided that the amplitude of the thread edge is approximately 0.05 to 0.15 times the thread height. The harder and more resistant the material is, the smaller the amplitude should be (and vice versa). Furthermore, for use as a “universal screw” for use with soft and hard materials, the amplitude may also be approximately 0.1 to 0.3 times the thread height.
In the screws of this invention, a further advantageous measure relates to the radially measured depth of the indentations. For use for screwing into softer materials, the depth of the indentations is preferably equal to our greater than 0.8 time the thread height. This factor may advantageously be approximately 0.8, but also may tend toward 1.0. For harder materials, the radial depth of the indentations is preferably approximately 0.2 to 0.3 times the thread height. For universal use, the depth may also be approximately 0.3 to 0.8 times the thread height.
The number of wave crests and wave troughs per turn of the thread, i.e. the circumferential angular spacing or pitch angle of the wave crests, also has a further influence on the properties of the screw of this invention. For use for screwing into softer materials, the pitch angle should lie in the range from 30° to 45°, resulting in a number n of 8 to 12 wave crests or wave troughs per turn of the thread (360°). For use in the case of harder materials, the pitch angle preferably lies in the range from 15° to 24°, resulting in a number n of 15 to 24 wave crests or troughs. For a design as a “universal screw” for soft and hard materials, the pitch angle may lie in the range from 20° to 35° (n=10 to 18).
In particular in conjunction with one or more of the features explained, it is advantageous if the thread, configured in practice as a one-start thread, has a lead which is approximately 0.5 times the outer thread diameter (nominal screw diameter). This achieves an increased thrust for quicker screwing in. Nevertheless, a high unscrewing torque is ensured for durable screwing prestress.
Further advantageous configurations of the invention are contained in further claims and the description which follows.
It should be noted at this point that all the features and measures described here can be used independently of one another or else in any possible or meaningful combination with one another.